Infrared Imaging Using Thermally Stable HgTe/CdS Nanocrystals.

Transferring nanocrystals (NCs) from the laboratory environment toward practical applications has raised new challenges. HgTe appears as the most spectrally tunable infrared colloidal platform. Its low-temperature synthesis reduces the growth energy cost yet also favors sintering. Once coupled to a read-out circuit, the Joule effect aggregates the particles, leading to a poorly defined optical edge and large dark current. Here, we demonstrate that CdS shells bring the expected thermal stability (no redshift upon annealing, reduced tendency to form amalgams, and preservation of photoconduction after an atomic layer deposition process). The electronic structure of these confined particles is unveiled using k.p self-consistent simulations showing a significant exciton binding energy of ∼200 meV. After shelling, the material displays a p-type behavior that favors the generation of photoconductive gain. The latter is then used to increase the external quantum efficiency of an infrared imager, which now reaches 40% while presenting long-term stability.

Multiresonant Grating to Replace Transparent Conductive Oxide Electrode for Bias Selected Filtering of Infrared Photoresponse.

Optoelectronic devices rely on conductive layers as electrodes, but they usually introduce optical losses that are detrimental to the device performances. While the use of transparent conductive oxides is established in the visible region, these materials show high losses at longer wavelengths. Here, we demonstrate a photodiode based on a metallic grating acting as an electrode. The grating generates a multiresonant photonic structure over the diode stack and allows strong broadband absorption. The obtained device achieves the highest performances reported so far for a midwave infrared nanocrystal-based detector, with external quantum efficiency above 90%, detectivity of 7 × 1011 Jones at 80 K at 5 µm, and a sub-100 ns time response. Furthermore, we demonstrate that combining different gratings with a single diode stack can generate a bias reconfigurable response and develop new functionalities such as band rejection.

Inside a nanocrystal-based photodiode using photoemission microscopy.

As nanocrystal-based devices gain maturity, a comprehensive understanding of their electronic structure is necessary for further optimization. Most spectroscopic techniques typically examine pristine materials and disregard the coupling of the active material to its actual environment, the influence of an applied electric field, and possible illumination effects. Therefore, it is critical to develop tools that can probe device in situ and operando. Here, we explore photoemission microscopy as a tool to unveil the energy landscape of a HgTe NC-based photodiode. We propose a planar diode stack to facilitate surface-sensitive photoemission measurements. We demonstrate that the method gives direct quantification of the diode's built-in voltage. Furthermore, we discuss how it is affected by particle size and illumination. We show that combining SnO2 and Ag2Te as electron and hole transport layers is better suited for extended-short-wave infrared materials than materials with larger bandgaps. We also identify the effect of photodoping over the SnO2 layer and propose a strategy to overcome it. Given its simplicity, the method appears to be of utmost interest for screening diode design strategies.

Mid-wave infrared sensitized InGaAs using intraband transition in doped colloidal II-VI nanocrystals.

Narrow bandgap nanocrystals (NCs) are now used as infrared light absorbers, making them competitors to epitaxially grown semiconductors. However, these two types of materials could benefit from one another. While bulk materials are more effective in transporting carriers and give a high degree of doping tunability, NCs offer a larger spectral tunability without lattice-matching constraints. Here, we investigate the potential of sensitizing InGaAs in the mid-wave infrared throughout the intraband transition of self-doped HgSe NCs. Our device geometry enables the design of a photodiode remaining mostly unreported for intraband-absorbing NCs. Finally, this strategy allows for more effective cooling and preserves the detectivity above 108 Jones up to 200 K, making it closer to cryo-free operation for mid-infrared NC-based sensors.

Lithium-Ion Glass Gating of HgTe Nanocrystal Film with Designed Light-Matter Coupling.

Nanocrystals' (NCs) band gap can be easily tuned over the infrared range, making them appealing for the design of cost-effective sensors. Though their growth has reached a high level of maturity, their doping remains a poorly controlled parameter, raising the need for post-synthesis tuning strategies. As a result, phototransistor device geometry offers an interesting alternative to photoconductors, allowing carrier density control. Phototransistors based on NCs that target integrated infrared sensing have to (i) be compatible with low-temperature operation, (ii) avoid liquid handling, and (iii) enable large carrier density tuning. These constraints drive the search for innovative gate technologies beyond traditional dielectric or conventional liquid and ion gel electrolytes. Here, we explore lithium-ion glass gating and apply it to channels made of HgTe narrow band gap NCs. We demonstrate that this all-solid gate strategy is compatible with large capacitance up to 2 µF·cm-2 and can be operated over a broad range of temperatures (130-300 K). Finally, we tackle an issue often faced by NC-based phototransistors:their low absorption; from a metallic grating structure, we combined two resonances and achieved high responsivity (10 A·W-1 or an external quantum efficiency of 500%) over a broadband spectral range.

Mapping the Energy Landscape from a Nanocrystal-Based Field Effect Transistor under Operation Using Nanobeam Photoemission Spectroscopy.

As the field of nanocrystal-based optoelectronics matures, more advanced techniques must be developed in order to reveal the electronic structure of nanocrystals, particularly with device-relevant conditions. So far, most of the efforts have been focused on optical spectroscopy, and electrochemistry where an absolute energy reference is required. Device optimization requires probing not only the pristine material but also the material in its actual environment (i.e., surrounded by a transport layer and an electrode, in the presence of an applied electric field). Here, we explored the use of photoemission microscopy as a strategy for operando investigation of NC-based devices. We demonstrate that the method can be applied to a variety of materials and device geometries. Finally, we show that it provides direct access to the metal-semiconductor interface band bending as well as the distance over which the gate effect propagates in field-effect transistors.

Helmholtz Resonator Applied to Nanocrystal-Based Infrared Sensing.

While the integration of nanocrystals as an active medium for optoelectronic devices progresses, light management strategies are becoming required. Over recent years, several photonic structures (plasmons, cavities, mirrors, etc.) have been coupled to nanocrystal films to shape the absorption spectrum, tune the directionality, and so on. Here, we explore a photonic equivalent of the acoustic Helmholtz resonator and propose a design that can easily be fabricated. This geometry combines a strong electromagnetic field magnification and a narrow channel width compatible with efficient charge conduction despite hopping conduction. At 80 K, the device reaches a responsivity above 1 A·W-1 and a detectivity above 1011 Jones (3 µm cutoff) while offering a significantly faster time-response than vertical geometry diodes.

Vanishing Confinement Regime in Terahertz HgTe Nanocrystals Studied under Extreme Conditions of Temperature and Pressure.

While HgTe nanocrystals (NCs) in the mid-infrared region have reached a high level of maturity, their far-infrared counterparts remain far less studied, raising the need for an in-depth investigation of the material before efficient device integration can be considered. Here, we explore the effect of temperature and pressure on the structural, spectroscopic, and transport properties of HgTe NCs displaying an intraband absorption at 10 THz. The temperature leads to a very weak modulation of the spectrum as opposed to what was observed for strongly confined HgTe NCs. HgTe NC films present ambipolar conduction with a clear prevalence of electron conduction as confirmed by transistor and thermoelectric measurements. Under the application of pressure, the material undergoes phase transitions from the zinc blende to cinnabar phase and later to the rock salt phase which we reveal using joint X-ray diffraction and infrared spectroscopy measurements. We discuss how the pressure existence domain of each phase is affected by the particle size.

The complex optical index of PbS nanocrystal thin films and their use for short wave infrared sensor design.

As nanocrystals (NCs) gain maturity, they become central building blocks for optoelectronics in devices such as solar cells and, more recently, infrared focal plane arrays. Now that the proof of concept of these devices has been established, their optimization requires a deeper understanding of their electronic and optical features to engineer their optoelectronic properties accurately. Though PbS NCs have been extensively investigated, the complex optical index of PbS NC thin films remains mostly unknown. Some previous works have unveiled the optical index for this type of material optimized for solar cells (excitonic peak at 940 nm), but longer wavelengths remain scarce and surface chemistry effects, which are known to be of central importance for layer doping, are simply unexplored. Here, we conduct a systematic investigation of the complex optical index of PbS NC thin films using broadband spectrally resolved ellipsometry. The obtained results are then compared with simulations combining tight-binding (TB) modeling at the NC level and the Bruggeman model to expand the results to the film scale. While TB calculation gives the NC optical indices, we extract the key NC film parameters such as the NC volume fraction and ligand indices by fitting the Bruggeman formula to ellipsometry measurements. We also bring evidence that this joint modeling method can be conducted without the need for ellipsometry data while preserving the main feature of the experimental results. Finally, the unveiled optical indices are used to model the absorption of short-wave infrared diode stacks based on PbS NCs and are relevant for state-of-the-art devices. Our electromagnetic modeling shows that the absorption within the contact is now a major limitation of the current device operated at the telecom wavelength.

Bias Tunable Spectral Response of Nanocrystal Array in a Plasmonic Cavity.

Nanocrystals (NCs) have gained considerable attention for their broadly tunable absorption from the UV to the THz range. Nevertheless, their optical features suffer from a lack of tunability once integrated into optoelectronic devices. Here, we show that bias tunable aspectral response is obtained by coupling a HgTe NC array with a plasmonic resonator. Up to 15 meV blueshift can be achieved from a 3 µm absorbing wavelength structure under a 3 V bias voltage when the NC exciton is coupled with a mode of the resonator. We demonstrate that the blueshift arises from the interplay between hopping transport and inhomogeneous absorption due to the presence of the photonic structure. The observed tunable spectral response is qualitatively reproduced in simulation by introducing a bias-dependent diffusion length in the charge transport. This work expands the realm of existing NC-based devices and paves the way toward light modulators.

Correlating Structure and Detection Properties in HgTe Nanocrystal Films.

HgTe nanocrystals (NCs) enable broadly tunable infrared absorption, now commonly used to design light sensors. This material tends to grow under multipodic shapes and does not present well-defined size distributions. Such point generates traps and reduces the particle packing, leading to a reduced mobility. It is thus highly desirable to comprehensively explore the effect of the shape on their performance. Here, we show, using a combination of electron tomography and tight binding simulations, that the charge dissociation is strong within HgTe NCs, but poorly shape dependent. Then, we design a dual-gate field-effect-transistor made of tripod HgTe NCs and use it to generate a planar p-n junction, offering more tunability than its vertical geometry counterpart. Interestingly, the performance of the tripods is higher than sphere ones, and this can be correlated with a stronger Te excess in the case of sphere shapes which is responsible for a higher hole trap density.